Update path of the ovn extensions doc
Update path of the ovn extensions doc

It was changed after migration of the ovn to neutron repo but
path in the comment wasn't changed.

TrivialFix

Change-Id: Ie4d7f0b85fe8997ea695fb633202c730a31a9b63

openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron,mahak/neutron,openstack/neutron